Prior round stripped provider prefixes from both sides and matched bare-only,
which over-matched: openai/gpt-4o would match default openrouter/gpt-4o. Now the
matcher compares bare model ids AND rejects when both sides identify DIFFERENT
providers (from provider/ prefix, @provider: qualifier, or the explicit provider
arg). Same-provider / unknown-session-provider still match. Added cross-provider
rejection regression tests.
Co-authored-by: allenliang2022 <allenliang2022@users.noreply.github.com>